In tools of this kind, vibrations are generated by the drive unit, or in other words by the internal combustion engine and the elements that are moved therewith in a rotational or translational manner, and these vibrations are at least partly transmitted to the handle or handles of the tool as well. As a result, the person using the tool experiences vibration to a sometimes considerable extent, which can be not only unpleasant but even unhealthy. It is well known that these vibrations at the handles also affect the work output, because especially when work extends over long periods, the user's hands and arms tend to become cramped. For a long time, there have therefore been attempts to keep the drive unit vibrations as low as possible and to insulate the handle elements from vibration to the greatest possible extent. To this end, the drive unit is as a rule supported in a damped manner with respect to the housing or at least with respect to the handle or handles; this is attained by anti-vibration elements, that is, vibration dampers.
The vibration dampers usually comprise elastic material and are disposed between the exciter system and the handles or housing of the tool to attain maximum possible vibration damping. Despite numerous experiments, it has been found, however, that virtually complete insulation of the handles from vibration is impossible, and that the vibration insulation attained is not completely satisfactory, so that the tool user is still subjected to considerable stress from these vibrations, which although diminished continue to be present.
To reduce these vibrations still further, the oscillating masses of the engine have been reduced to the greatest possible extent by means of suitable structural configuration (lightweight construction) and by a suitable selection of material (light metal and light metal alloys). By this means, it has been possible to lessen the vibration in the handle element, but not to eliminate it.
To increase vibration insulation between the exciter system (drive unit) and the handles, the damping members, which were made of rubber or steel springs, have been replaced by softer elements. By this means, although it was possible to increase the degree of insulation considerably, still this had the disadvantage that the required guidance of the tool was impaired. The damping elements were therefore adapted to be harder, but in adaptation a compromise between accurate guidance and vibration damping must always be made. Although hard dampers enable good, direct guidance of the tool by the operator, they transmit a considerable portion of the vibration originating in the drive unit, while with soft dampers this situation is precisely the opposite.
According to prevailing opinion thus far, the vibrations transmitted to the handle are substantially dependent on the quality of balancing of the drive unit. In this respect the attempt is made to keep imbalance of the rotating structural parts of the engine as low as possible [on this aspect, see also VDI-Richtlinie 2060 (German Engineers Association Guideline 2060)], and for this reason, two well-defined crank arms are for instance provided on the crankshaft, the piston rod being pivotably connected between them. The balancing weights in the crank webs serve to equalize the inertial forces of the piston at top dead center; they are therefore arranged such that they counteract these forces. The crankshaft itself, along with the elements mounted on it, such as a fan wheel and clutch, are also balanced statically and dynamically as much as possible.